The present invention relates generally to an illumination optical system and an exposure apparatus using the same, and more particularly to an illumination optical system that uses a light source in an extreme ultraviolet (“EUV”) or X-ray region with a wavelength between 10 nm and 200 nm, and an exposure apparatus that uses the illumination optical system to expose an object, such as a single crystal substrate for a semiconductor wafer, and a glass plate for a liquid crystal display (“LCD”).
A reduction projection exposure method that uses, for example, EUV light or X-ray have been proposed as one of methods for manufacturing semiconductor circuit devices having a fine pattern (see, for example, Japanese Patent Application Publication No. 10-70058 (or its U.S. counterpart, U.S. Pat. No. 6,504,896B1), Japanese Patent Application Publication No. 2003-045774 (or its U.S. counterpart, U.S. Patent Application Publication No. 2003031017A1), and Japanese Patent Application Publication No. 2003-045784 (or its U.S. counterpart, U.S. Patent Application Publication No. 2003031017A1)).
This method uses the EUV light to illuminate a mask (or a reticle) that forms a circuit pattern, and projects a reduced size of the pattern on the mask, onto a wafer surface, and to expose photoresist on the wafer surface for pattern transfer.
FIG. 11 schematically shows principal part of a conventional EUV reduction projection exposure apparatus 1000. In FIG. 11, 1001 denotes an EUV light emission point, 1002 denotes EUV light, 1003 denotes a filter, 1004 denotes a first rotational paraboloid mirror, 1005 denotes a reflection integrator, 1006 denotes a second rotational paraboloid mirror, 1007 denotes a reflection mask, 1008 denotes plural mirrors that constitute a projection optical system, 1009 denotes a wafer, 1010 denotes a mask stage, 1011 denotes a wafer stage, 1012 denotes an arc aperture, 1013 denotes a laser light source, 1014 denotes a laser condensing optical system, and 1017 denotes a vacuum chamber. FIG. 13 is a plane view showing a relationship between an illuminated area 1015 on the mask 1007 and an arc area 1016 to be exposed.
The exposure apparatus 1000 thus includes a light source section 1013, 1014 that generates the EUV light, an illumination optical system (i.e., the first rotational paraboloid mirror 1004, the reflection integrator 1005 and the second rotational paraboloid mirror 1006), the reflection mask 1007, the projection optical system 1008, the wafer 1009, the mask mounted stage 1010, the wafer mounted stage 1011, an alignment mechanism (not shown) for precise alignment between mask and wafer positions, the vacuum chamber 1017 that maintains vacuum of the entire optical system vacuum for reduced attenuations of the EUV light, and an exhaust apparatus (not shown).
The illumination optical system uses the first rotational paraboloid mirror 1004 to condense the EUV light 1002 from the emission point 1001 into the reflection integrator 1005 so as to form secondary light sources, and uses the second rotational paraboloid mirror 1006 to superimpose and condense the EUV light from these secondary light sources so as to uniformly illuminate the mask 1007.
The reflection mask 1007 forms a pattern to be transferred, using a non-reflected part made of an EUV absorber on a multilayer mirror. The projection optical system 1008 images, on the wafer 1009, the EUV light that has information of a circuit pattern reflected by the reflection mask 1007.
The projection optical system 1008 is configured to have excellent imaging performance in an off-axis, thin arc area (i.e., apart from an optical-axis center). The aperture 1012 with the arc opening just prior to the wafer 1009 enables exposure to use only this thin arc area. The exposure scans the reflection mask 1007 and the wafer 1009 simultaneously and transfers a rectangular shot on the entire surface of the mask.
The projection optical system 1008 is comprised of plural multilayer mirrors, and configured to project a reduced size of pattern on the mask 1007 onto the wafer 1009 surface. The projection optical system 1008 typically forms an image-side telecentric system, and usually provides an object side (or the reflection mask side) with a non-telecentric structure so as to avoid physical interference with the illumination light incident upon the reflection mask 1007.
The laser condensing optical system 1014 condenses a laser beam from the laser light source 1013 onto a target (not shown) at the emission point 1001, generating a high-temperature plasma light source 1001. The EUV light 1002 thermally radiated from this plasma light source is reflected on the first rotational paraboloid mirror 1004 and turns into parallel EUV light. This light is reflected on the reflection integrator 1005 and forms a multiplicity of secondary light sources.
The EUV light from these secondary light sources is reflected on the second rotational paraboloid mirror 1006 and illuminates the reflection mask 1007. Distances from the secondary light sources to the second rotational paraboloid mirror 1006 and from the secondary rotational paraboloid mirror 1006 to the reflection mask 1007 are set to be equal to a focal distance of the second rotational paraboloid mirror 1006.
Since a focal point of the second rotational paraboloid mirror 1006 is located at positions of the second light sources, the EUV light emitted from the secondary light sources irradiates as parallel light the reflection mask 1007. The projection optical system 1008 is configured to project an image of the secondary light sources onto an entrance pupil surface, and thereby meets the Kohler's illumination conditions. The EUV light that illuminates one point on the reflection mask 1007 is superimposed EUV beams emitted from all the secondary light sources.
The illuminated area 1015 on the mask surface is similar, as shown in FIG. 12, to a plane shape of a concave or convex mirror as an element in the reflection integrator 1005, and it is an approximately rectangular shape that includes the arc shape 1016 to be actually exposed. The projection optical system 1008 is configured to project an image of the secondary light sources onto its pupil surface.
However, the conventional EUV reduction projection exposure apparatus has been disadvantageous, because the rotational paraboloid mirror 1004 has a reflective surface asymmetrical to the optical axis of the EUV light 1002 isotropically emitted from the light source 1001, and cannot uniformly illuminate the reflection integrator 1005. As a result, an angular distribution of light for illuminating the mask 1007 becomes non-uniform, and pattern resolving power deteriorates.